The present study aims to estimate the wind ventilation performance for pedestrian level domains from the air quality point of view. Four typical models of a densely built-up area were considered and numerically simulated in order to examine the effects of the geometry of such models on wind flow characteristics, which in turn affect the air quality, within the pedestrian domain of a street located within this area. The calculated flow fields were employed to estimate the exceedance probabilities within the study domain using air exchange rate within the domain. The air exchange rate represents air quality. The study has been applied to nine cities in Japan: Tokyo, Osaka, Sapporo, Niigata, Fukuoka, Nagoya, Sendai, Yokohama, and Kyoto, based on their mean wind velocity data. The results demonstrated that the exceedance probability analysis of the pedestrian wind environment is very important during the design stage of inhabited areas for the evaluation of pollutant-removal efficiency by the applied wind. Also, the calculated probabilities demonstrated substantial dependence on both the geometry of building arrays and the wind conditions of the nine cities. [This abstract is not included in the PDF]
This study investigated the effect of absorptive building materials in decreasing indoor concentration of formaldehyde by numerical simulation. Many studies have been reported on this subject, but most were limited to the investigation of one room. In this study, the numerical analysis models were based on the standard house of Architectural Institute of Japan to study the building material in full-size house. The performances of absorptive materials were compared by changing the surface area and position, and the most effective arrangement was studied. [This abstract is not included in the PDF]
The residence time of indoor air in upper room zone which can be derived by L-PFR and VF was calculated to evaluate the germicidal efficacy of upper room UVGI system. Models which have a dimension of 3.5X3.0X2.5[m]and 3 different types of supply and exhaust openings and several supply air rates, were simulated with CFD and compared to each other. As results, PFR, VF, and consequently residence time of UVGI zone changed apparently as the amount of supply air increased or the type of supply and exhaust openings changed. The design of supply and exhaust openings should be considered for the efficacy of upper room UVGI system.
The ventilation potential was investigated for a residential basement with an attached areaway in order to ensure effective air exchange rate. For this purpose, it is necessary to make the assessment of ventilation performances for the interior space of basement as well as the exterior space of areaway, taking the annual characteristics of local wind into account. In this study, the method of exceedance probabilities analysis was applied based on the local ventilation index calculated by CFD simulation for 16 wind incidences. It was found that the ventilation rate changed a lot for different building orientations. The results also showed that the orientation with maximum ventilation rate was different for basement and areaway space. [This abstract is not included in the PDF]
In a recent study, a formulation of concentration response factor method was established. The method is able to calculate three-dimensional transient pollutant transport in much shorter time than CFD in steady state flow field. In general, buildings are composed of multiple rooms, and the structure of airflow might be regarded as a network. Then, we develop a model that couples three-dimensional transient pollutant transport in a room into a flow network mode. In the coupling model, the air transports between each room are calculated using a flow network model, and the three-dimensional transient pollutant transports between each element (e.g. contaminant generation point, opening, air outler, air inlet,) are calculated using a method based on the concentration response factor.
The objective of this paper is to find out the influence of the cell geometry on the numerical errors. More precisely, the effect of two cell types is investigated, namely, the hexahedral cells and the tetrahedral cells. The truncation errors are compared between the two mesh styles and it is proved that the tetrahedral-based mesh gives higher truncation error compared to the hexahedral-based mesh. Moreover, the effect of mesh non-orthogonality is investigated and it is found that it gives higher truncation errors. The flow problem of unsteady flow around square 2D cylinder is solved on both mesh systems for similar density using LES to give useful insight into the accuracy of the two mesh systems considered. [This abstract is not included in the PDF]
The heat island phenomenon and urban thermal environmental degradation become serious problems in Japan. Although various relaxation measures for thermal environment have been proposed, the effects of them are different according to urban configuration. For proper urban design, it is necessary to understand quantitatively the effects of the measures. Therefore, in this paper, coupled simulation of convection, radiation and conduction was done in Otemachi area, as a high-rise office block, and in Kyobashi area, as a middle-rise office block in Tokyo, to compare the effects of the various relaxation measures. The result showed that the measures of high albedo material and greening on ground and road are effective in Otemach, and heat release point and way of air-conditioning affect very much in Kyobashi.
Numerical simulation was carried out by means of a three-dimensional hydrodynamic-ecosystem coupled model in order to analyze the material cycle during March 2005 and September 2007 in Lake Biwa. It was revealed that the vertical mixing of waters was considerably delayed in the winter during 2006 and 2007 due to warm winter. Consequently, the concentration of dissolved oxygen was not recovered enough to the saturated level around the water bottom. Additionally, the numerical model predicted that nutrients were accumulated above the water bottom of Northern Basin in Lake Biwa since they were not transported to Southern Basin. [This abstract is not included in the PDF]
The fields of flow velocity and water temperature in Lake Kitaura were simulated by means of a three-dimensional hydrostatic model. The main purpose of the present study was to reproduce the stratification structure in Lake Kitaura during March and September 2006. As a result of numerical simulation, the hydrostatic model reproduced the long-term stratification in June and July, the diurnal stratification in August, and the disappearance of stratification in September. The vertical circulation of waters was then formed by the wind stress on the water surface, and its feature was expected to depend on the stratification structure. [This abstract is not included in the PDF]
The non-hydrostatic 3-dimensional physical model was developed to predict physical phenomenon of small shrimp cultivation pond in which several paddle wheels are installed to supply oxygen and to collect the sludge at the center of the pond. As a result, the disappearance of stratification and the current field were reproduced, considering the effects of paddle wheels as the boundary conditions of numerical simulation. Such phenomena were also observed in the shrimp cultivation pond in Amakusa of Kyusyu District, therefore the non-hydrostatic 3-dimensional physical model approximately captured the physical phenomena in the cultivation pond with paddle wheels. [This abstract is not included in the PDF]
This paper describes large-scale LES computation on the underbody flow with semi-complex underbody configuration using 22-23 million elements. LES results obtained well reproduced the similarity law on energy spectrum in the inertial sub-range, showing applicability of the LES calculations in this study. Difference in flows with flat and semi-complex underbody configurations and effects of moving ground and rotating wheels were clarified. Effects of the underbody configuration on the underbody and wake flows were shown and also CD value calculated was discussed. [This abstract is not included in the PDF]
A transient, non-isothermal three-dimensional numerical model has been developed to simulate proton exchange membrane fuel cells. The flow behavior in the gas channel and porous GDL has been investigated by analyzing the pressure and velocity distribution. The effect of GDL properties, such as porosity and permeability, on the flow behavior will also be observed. Besides this, the effect of clamping force on the GDL deformation and its effect on the performance of polymer electrolyte fuel cell will be studied. [This abstract is not included in the PDF]
In this study, we have conducted a numerical study on the behaviors of flame in a channel influenced by ignition zone and preheating phenomenon. A system of two-dimensional governing equations for reacting flows, evolved through a single step irreversible chemical reaction between ethylene and oxygen, is solved by using Front flow Red based on the low Mach number approximation. It is found that both ignition zone and preheating phenomenon are very sensitive to the shapes of flame and their propagation speed. [This abstract is not included in the PDF]
Effects of particle volume fraction on drag force on solid particles falling in the turbulent air have been investigated experimentally. A change of drag force is assessed by the change of particle velocity in airflow. In still flow, as particle volume fraction increases (up to O (10-2)) the drag force for each particle increases compared to prediction using the drag coefficient by Shiller & Naumann (1933). While in a turbulent flow (Stokes number is O (103)), the drag force for each particle decreases, but a change of drag force is relatively insensitive to particle volume fraction or turbulent intensity. [This abstract is not included in the PDF]
Due to the high computational cost, well-resolved large-eddy simulation (LES) of wall-bounded turbulent flow at very high Reynolds number (>107) is still difficult. To deal with this issue, Detached-eddy simulation (DES) is used in engineering applications. In this study, DES based on Spalart-Allmaras (SA) model was conducted in several turbulent flows. The performance of the DES-SA model is validated in fully developed turbulent channel flow, turbulent flow around a circular cylinder and turbulent flow around a NACA0012 airfoil. The advantages and disadvantages of the DES-SA model are summarized. Possible approaches to improve the DES-SA model are discussed. [This abstract is not included in the PDF]
Large eddy simulation of internal flows of a mixed-flow pump is performed. The pump has open impeller blades and diffuser vanes. The objective of this research is to verify the accuracy of the pump performance prediction by LES with a particular emphasis placed on the instability characteristics. In this paper, we show the preliminary results by LES, in which the computational grids do not resolve turbulent boundary layer developed on blades surfaces. The LES successfully predicted the instability characteristics. By analyzing computed flow fields, we also investigated the detailed mechanism that leads to the instability characteristics. [This abstract is not included in the PDF]
In the core region of spanwise rotating channel flows, the mean velocity profile is approximately linear with a slope of twice the system rotation rate. The mechanism of this zero mean absolute vorticity state is investigated from the turbulence modeling point of view. The mean velocity profile is calculated using three simple nonlinear eddy-viscosity models. It is shown that two models taking into account the effect of the advection of the Reynolds stress well reproduce the zero mean absolute vorticity profile. In particular, the latter model reflects the memory effect of the second-order nonlinear term. This effect is confirmed by an analysis using the Green's function for the velocity fluctuation. [This abstract is not included in the PDF]
Eddy viscosity in magnetohydrodynamic (MHD) turbulence is investigated with the notion ofsynthesized timescale composed of the eddy-turnoverand the Alfvén times. For the purpose of determiningthe timescale weight factor, we fully utilize the factthat the decay rate of the turbulent MHD residualenergy, the difference between the kinetic andmagnetic fluctuation energies, is constituted by theeddy-distortion and the Alfvén effects. Comparison ofthe spacecraft observations of solar-wind turbulenceand the simulation of the turbulence modelincorporating the residual-energy equation provides uswith the estimate of the timescale weight factor. Thespontaneous flow generation observed in the reversedmagnetic shear (RS) mode in tokamak plasma isexamined by using the numerical simulation of aturbulence model. Flow localization, which could notbe reproduced with the usual eddy viscosity, issuccessfully reproduced with the synthesized viscosity. [This abstract is not included in the PDF]
If an LES of turbulent channel flow is carried out using low-order finite difference method with an eddy viscosity model such as the Smagorinsky model, it is known that the turbulent energy and the anisotropy of velocity fluctuation is greater than those of DNS. In this study, to improve the points, a low-order scale similarity model (SSM) is adopted. As a result, the SSM in all simulation cases can decrease the turbulent energy but cannot improve the overestimated anisotropy. [This abstract is not included in the PDF]